Heat generating systems with furnaces for combusting fossil fuels have long been employed to generate controlled heat, with the objective of doing useful work. The work might be in the form of direct work, as with kilns, or might be in the form of indirect work, as with steam generators for industrial or marine applications or for driving turbines that produce electric power. Modern water-tube furnaces for steam generation can be of various types including fluidized-bed boilers. While there are various types of fluidized-bed boilers, all operate on the principle that a gas is injected to fluidize solids prior to combustion in the reaction chamber. In circulating fluidized-bed (CFB) type boilers a gas, e.g., air, is passed through a bed of solid particles to produce forces that tend to separate the particles from one another. As the gas flow is increased, a point is reached at which the forces on the particles are just sufficient to cause separation. The bed then becomes fluidized, with the gas cushion between the solids allowing the particles to move freely and giving the bed a liquid-like characteristic. The bulk density of the bed is relatively high at the bottom and decreases, as it flows upward through the reaction chamber where fuel is combusted to generate heat.
The solid particles forming the bed of the circulating fluidized bed boiler typically include fuel particles, such as crushed coal or other solid fuel, and sorbent particles, such as crushed limestone, dolomite or other alkaline earth material. Combustion of the fuel in the reaction chamber of the boiler produces flue gas and ash. During the combustion process, the sulfur in the fuel is oxidized to form sulfur dioxide (SO2), which is mixed with the other gasses in the furnace to form the flue gas. The ash consists primarily of unburned fuel, inert material in the fuel, and sorbent particles, and is sometimes referred to as bed materials or re-circulated solids.
The ash is carried entrained in the flue gas in an upwardly flow and is exhausted from the furnace with the hot flue gas. While entrained therein and being transported by the flue gas, the sorbent particles that are present within the reaction chamber, i.e., furnace or combustor, capture, i.e., absorb, sulfur from the SO2 in the flue gas. This reduces the amount of SO2 in the flue gas that ultimately reaches the stack and as such the amount of SO2 that is exhausted into the environment.
In order to replenish the solid particle materials that are consumed in or exhausted by the furnace, fresh fuel and sorbent particles as well as recycled ash are continuously introduced to the bed of the circulating fluidized bed boiler. Continuing, after being exhausted from the furnace, the flue gas and ash are directed to a separator, such as a cyclone, to remove the ash from the flue gas. Two parallel paths are then typically provided for re-circulating the separated ash back to the bed of the circulating fluidized bed boiler. At any given time, the separated ash may be directed along either or both of said parallel paths by a solids flow control valve located between the separator and said two parallel paths. Such solid flow control valves are well known in the art and may be controlled pneumatically, hydraulically or in some other functionally equivalent manner.
Circulating fluidized bed boilers are designed so as to operate within a narrow temperature range in order to thereby promote the combustion of fuel, the calcination of limestone and the absorption of sulfur. This narrow range of furnace temperatures must be maintained over a range of furnace loads, from full load down to some level of partial loading. The furnace temperature is controlled through absorption of heat from the flue gas and bed ash that is produced as a result of combustion in the reactor chamber of the furnace. While most of the heat absorption is through the furnace walls and the in-furnace panels, on larger circulating fluidized bed boilers, heat absorption by the furnace enclosure walls and in-furnace panels is insufficient to achieve the desired operating temperatures. For these larger circulating fluidized bed boilers, therefore, external heat exchangers are employed to absorb heat from the ash that is removed from the flue gas in the cyclone or other separator, before the ash is re-circulated to the to the circulating fluidized bed boiler. Such external heat exchangers are commonly referred to as External Heat Exchangers (EXE) or Fluid Bed Heat Exchangers (FBHEs).
Accordingly, if directed along one of the two parallel re-circulating paths, the sorbent and other ash particles are fluidized and these fluidized ash particles are then transported to and are made to flow through a FBHE by means of injected high pressure gas, e.g., air, which is normally at a pressure of about 200 inches water gage (WG). Heat is transferred from the fluidized particles to a working fluid such as water, steam, a mixture of both or some other coolant flowing through a tube bundle within the FBHE. The flow of cooled fluidized particles is then reintroduced into the furnace. The amount of cooling of the fluidized particles that is performed in the FBHE is typically controlled based on the gas temperature within the furnace that is desired.
If directed along the other one of the two parallel re-circulating paths, the sorbent and other ash particles are also fluidized and are entrained therewithin and are transported by an injected high pressure gas, such as air, again normally at a pressure of around 200 inches WG. In this case, in accordance with this path, the fluidized particles are directed through an ash re-circulation pipe having a seal, commonly referred to as a seal pot or siphon seal, that is suitably installed so as to be operative to ensure proper flow of gas and ash in the primary loop, which is defined as the furnace, the separator, i.e., cyclone, seal pot and FBHE. The seal pot functions to prevent a backflow of gas and solid particles from the furnace into the re-circulation pipe. From the seal pot, the sorbent and other solid ash particles are then reintroduced into the furnace without being cooled.
U.S. Pat. Nos. 6,779,492 and 6,938,780, which are also assigned to the same assignee as that of all of the rights in the present application, provide detailed descriptions of conventional circulating fluidized bed boilers having seal pots and FBHEs.
There remains a need for a more efficient and less expensive means for recycling ash in circulating fluidized bed boiler heat generating systems. For example, it would be beneficial if the relatively high pressure fluidizing air required by conventional FBHEs and seal pots could be eliminated, since this would reduce not only the expense of providing the required high pressure blowers and fluidizing nozzles of conventional construction, but also would reduce the dynamic loading to which the structural steel, which is required to support the FBHEs and seal pots of conventional construction is subjected, and in addition the consumption as well of power that is required to operate such high pressure blowers in order to thereby provide the necessary supply of high pressure air. Additionally, it would be beneficial to have higher heat transfer rates in the FBHE than those that are now possible when FBHEs of conventional construction are employed. Heat transfer is typically defined y the equation Q=R×S×LMTD where Heat transferred (Q=Btu/hr), Heat Transfer Rate (R=Btu/hr−Ft2−F), Surface (S=Square Feet (Ft2)) and Log Mean Temperature Difference (LTMD=Deg. F). For a constant transfer rate (R), increasing the LMTD results in a reduction of required heat exchanger surface (S) for a given heat loading. The moving bed heat exchanger (MBHE) constructed in accordance with the present invention improves on the LMTD over that in typical FBHEs by permitting full counter-flow of solids and working fluid.